A number of emerging technologies are incorporating photonics. Among these are optical imaging, telecommunications, entertainment devices, image-projection systems, medical diagnosis and treatment, photolithography, materials inspection, biosensors, and surveillance. Applications of these technologies share a requirement for the rapid and accurate scanning of a laser beam either to image an object or to project light onto a surface.
When a dynamic system or process is imaged optically, the rate of image acquisition (i.e., number of image frames acquired per unit time) is an important consideration. A “dynamic system” may be, for example, a stationary object that changes over time, a specimen that moves spatially within a field-of-view, or both processes occurring simultaneously. Many important processes occur within time domains that are less than one second. In such cases, it is frequently desirable to acquire images in at least two spatial dimensions as rapidly as is consistent with the sampling of sufficient numbers of photons to form an acceptable image.
Many imaging applications of dynamic systems and processes also require optimal spatial resolution. Laser-scanning confocal microscopy is commonly used to improve this parameter, particularly in the z-dimension that typically extends parallel to the optical axis. In a scanning-microscope system, such as a laser-scanning confocal microscope system, the illuminating light or specimen must be moved relative to the other, or moved relative to one another. This can be accomplished by moving the specimen while keeping the illuminating light in a fixed position, by moving the illuminating light across the specimen while the latter is kept stationary, or by simultaneously moving both the illumination light and the specimen.
Certain optical advantages can be achieved by keeping the illumination light stationary and moving the specimen (for example, see U.S. Pat. No. 3,013,457, incorporated herein by reference, which provides an original description of a confocal optical system). However, this approach involves accelerating, moving, and decelerating the relatively large mass of a microscope stage or other type of inspection platform, which typically prevents scanning at rates greater than a few frames per second. In addition, this approach restricts or prevents the use of immersion objectives, in which an intermediate layer of an appropriate medium, such as oil, water, or glycerin, is maintained between the objective and the specimen.
Because of such limitations, it is common to scan the beam of illumination light (typically a laser beam) over the specimen in a two-dimensional raster manner (involving one-dimensional lines repeated with intervening steps in the orthogonal dimension) in the majority of modern scanning microscopes. The laser beam is scanned by a beam-steering device comprising multiple mirrors mounted on respective devices capable of controlled motion, such as galvanometers or piezoelectric elements, or using micro-mirrors mounted on microelectromechanical systems (MEMS). Another beam-steering approach utilizes stationary devices, such as acousto-optical beam deflectors (AODs) that exploit changes in refractive index of a material to alter the path of the light beam. However, each of these beam-steering devices is constrained by limitations related to their maximum achievable scan rates and/or optical properties.
Galvanometers are currently the beam-steering device most commonly employed in scanning optical systems. Respective mirrors, mounted on two independent galvanometers, are used to achieve beam steering in two (x and y) spatial dimensions. Closed-loop galvanometer pairs have been used most frequently; these devices exploit the ability to modulate and control the position of each mirror as it is moved back and forth in a single dimension in an accurate manner that is inherent to this type of device. A closed-loop galvanometer typically also has position-feedback signals that can be used to verify the position of the mirror at a given point in time. However, the frequency response of this type of galvanometer is limited (generally to less than 1 kHz) by several factors, and this limitation restricts the galvanometer's image-acquisition rate to typically less than video rates. These factors include the extent of mechanical movement of the mirror and the size (and hence the mass) of the reflective surface required. Ultimately, the time required to dissipate heat resulting from the electromagnetic forces used to drive movements of the mirror becomes limiting. All of these factors are inversely related to the frequency response of the galvanometer system.
In another approach, resonant galvanometers, which have lower-friction movements, can be driven at frequencies of up to 8 kHz. Such galvanometers have been used to deflect a laser beam in one spatial dimension. A slower (30-60 Hz) closed-loop galvanometer is used to deflect the beam in the second spatial dimension. Using this combination of galvanometers, acquisition rates of 30-60 frames/sec have been achieved for two-dimensional images. For examples, see Tsien and Bacskai, “Video-Rate Confocal Microscopy,” in Pawley (ed.), Handbook of Biological Confocal Microscopy, 2nd ed., chapter 29, Plenum Press, New York, 1995, and U.S. Pat. No. 5,283,433, incorporated herein by reference.
Since prior-art beam-steering systems utilize two mirrors to achieve both x- and y-direction scanning, these systems cannot place the axis of a primary deflection surface in a telecentric conjugate image plane. The need to utilize physically separate mirrors in galvanometer-based systems to steer the laser beam in two spatial dimensions in galvanometer-based systems imposes optical limitations (e.g., see the discussion by Stelzer, “The Intermediate Optical System of Laser-Scanning Confocal Microscopes,” in Pawley (ed.), Handbook of Biological Confocal Microscopy, 2nd ed., chapter 9, Plenum Press, New York, 1995). In imaging situations, in which laser-scanning confocal microscope systems utilizing single-photon excitation are used, it is necessary to sense light originating in the sample, such as fluorescent or reflected light, using a fixed-spot detector such as a photomultiplier tube or photodiode. To focus light from the sample onto a fixed point, the light must be de-scanned by the beam-steering device. Such de-scanning is optimal whenever the axis of the primary deflecting surface is placed at a telecentric conjugate image plane. However, such placement is not possible if, as in the prior art, separate reflective surfaces are used to deflect the beam in each of the two respective dimensions. Placement of the reflective surfaces in an axial parallel arrangement reduces, but does not eliminate, the associated optical distortion.
Another conventional approach to rapid, single-axis laser-beam deflection involves the use of an acousto-optical beam deflector (AOD). As noted previously, this device exploits induced changes in refractive index of a material to deflect the beam rapidly (with a 1-5 kHz frequency range) in one spatial dimension (e.g., x-dimension). As is the case for the resonant galvanometer, a second device is required to deflect the beam in the second spatial dimension (e.g., y-dimension). In addition, although scanning systems utilizing AOD devices have achieved high scan rates over a somewhat limited range of deflection angles, use of the AOD introduces optical disadvantages, particularly when used with laser-scanning confocal microscope systems. These disadvantages include reduced transmission efficiency, wavelength-dependent angles of deflection, and the inability of light emitted from the sample at wavelengths greater than that shone on the sample (e.g., fluorescence) to be de-scanned by the AOD device along the optical path used by the illuminating light. Additional optics are required to reduce the impact of these disadvantages on spatial resolution, which decreases the optical efficiency that can be achieved.
High-rate (1-10 kHz frequencies), 2-axis beam deflection has been achieved using an electrostatically actuated MEMS micro-mirror beam-steering device. However, the low level of torque produced by these devices limits the size of the reflective surface to typically <1 mm. Such a small clear aperture limits the achievable spatial resolution to much less than that of confocal systems currently available commercially and places important limitations on the properties of the intermediate optical system that can be used. Increasing the size of the mirror results in a marked reduction in scan frequency and an increase in the dynamic deformations of reflective surfaces. These deformations diminish the quality of the reflected light and, thus, the optical quality of acquired images.
Thus, there is currently a need for a two-axis beam-steering device having a single, large reflective surface.